JoelOnSoftware: "You know how an iceberg is 90% underwater? Well, most software is like that too -- there's a pretty user interface that takes about 10% of the work, and then 90% of the programming work is under the covers."

http://www.joelonsoftware.com/articles/fog0000000356.html

http://javangelist.snipsnap.org/space/Iceberg+Secret

See also: WikiIceberg.